1. Field of the Invention
This invention relates to scanning apparatus which may be used in a real-time imaging system, and has particular utility in a real-time passive millimetre wave imaging system, as well as at other wavelengths. The scanning imaging apparatus may also be used in other radiometry systems.
2. Description of Prior Art
Imaging using electromagnetic radiation at millimetre wavelengths, or thereabouts, is potentially useful as an all-weather surveillance and guidance aid and for indoor and outdoor security applications, but any practically useful system must be capable of imaging in real-time. This was until recently a problem due to the high number of very expensive receivers required that are able to operate at the frequencies of interest.
Imagers that operate at millimetre wavelengths or thereabouts often use a concave mirror or a lens to focus radiation from the scene being imaged onto an array of receivers. At present, large two-dimensional arrays of receivers which cover the whole of a required image are not available. Instead, a far smaller number of receivers is scanned across the image in order to build up the complete picture.
Current millimetre wave imaging systems use mechanical scanning of one or several channels to synthesise an image. Ultimately, electronic scanning and staring array techniques could be developed to implement real-time millimetre wave imaging, although there are several problems associated with such a solution. Firstly, as the wavelength is necessarily long, in order to image under adverse weather conditions the system aperture must be large to gain adequate resolution. In some millimetre wave imaging systems the input aperture may be of the order of 1 m in diameter. Secondly, the cost per channel is high so that any electronically scanned or staring array technique is likely to be prohibitively expensive. Furthermore, in the case of millimetre wave staring arrays there are fundamental problems analogous to the cold shielding problems encountered in infrared systems.
Another requirement of a practical millimetre wave imaging system is that it should ideally be able to operate at TV-compatible rates (i.e. 50 Hz for the UK, 60 Hz for the USA). In the infrared, scanning systems are often plane mirrors flapping about an axis contained within their surface. This is not a practical option in the millimetre waveband as large aperture mirrors would be required to flap back and forth at TV-compatible rates, requiring a large acceleration, and thus force at the end of each scan, and would therefore require a very large and complex mechanical arrangement.
In infrared imaging systems, where input apertures are typically only 10 mm in diameter, rotary systems have been used (EP 0226273). Furthermore, in the infrared, it is usual to employ a focal telescopes to match the field of view in the scene to that of a rotating polygon scanning means. This is impractical in high resolution millimetre wave imaging where the input apertures have considerably greater diameters and a focal telescopes would need to be excessively large.
Any scanning mechanism used in a millimetre wave imaging system must therefore be situated in either the object or the image plane. Furthermore, any scanning mechanism situated in the image plane must have good off-axis performance. This is difficult to achieve using existing technology.
Another known scanning method used in infrared imagers is a system of two discs rotating about axes which are slightly inclined to the normals to their faces (U.S. Pat. No. 4,923,263). Radiation incident on the first disc is reflected at oblique incidence from the first rotating disc and passes to the second disc to experience a second reflection. By varying the orientation and relative speed of rotation of the discs, varying scan patterns can be achieved. Such a two-axis rotating disc system would not be ideal for use in imagers operating at or around millimetre wavelengths, however, as the system would be inconveniently large.
Applicant's International patent No. WO2002/066998—the whole contents of which are hereby included by reference, discloses a scanning imaging system that operates on the principle of scanning a solid angle to be imaged past a relatively small number of sensors, this scanning being performed at a sufficient rate so as to produce a real-time video image of the scanned volume. The disclosure provides a means for generating a compact mechanically scanned system. This consists of a polarising grid reflector, a image plane of receivers in a linear array, a polarisation twisting quarter-wave plate or ferrite and a rotating tilted mirror. Radiation from the scene is transmitted through the grid, and reflected from the mirror—the rotation of which conically scans the beam—and then arrives back at the grid with an orthogonal polarisation to that which was incident on the imager. This radiation is then reflected by the grid and focussed onto the receiver array. The grid and mirror typically have aspheric shapes to correct for optical aberrations.
Because of the long wavelength as compared to optical systems, imagers such as that disclosed in WO2002/066998 are generally required to have large apertures to achieve a useful angular resolution. An aperture of around 1 m might be typical. To make such an imager deployable it must be as compact as possible for its aperture which has hitherto required a ‘fast’ optical system, with the focal length/aperture ratio typically around 0.5 (i.e. the f/number is f/0.5). This gives a system of relatively short overall length and minimises any overdimensioning (compared to the aperture) of the grid. Systems faster than f/0.5 are usually too hard to correct for the typical fields of view required.
To avoid producing imagery with gaps, the image plane should be sampled at least once per 3 dB spot size in the direction of the receiver array, and to avoid losing any information it is preferable that the image plane is sampled twice per 3 dB spot size (the Nyquist rate). This puts demands on the maximum size of the receiver array in the image plane, particularly when the desired wavelengths of operation are reduced. The receiver array is made up from a plurality of receiver elements, each of which comprises a receive antenna coupled to receive electronics. The receive antennas of the receive elements need to be positioned on the image plane so as to be in the optimal reception position, and so the sampling criteria described above puts a maximum value on the spatial separation of the receive element antennas in a given system.
This problem can be overcome to some extent by using a two-dimensional array of receivers, where the rows of receivers may be staggered, as described in WO2002/066998, and more particularly on pages 6-8 of that publication, but it is still difficult to sample an image well throughout the field of view and larger numbers of receivers are required than would be the case if a linear array were possible.